LED devices have been widely used as low-energy replacements for traditional light sources. In particular, with the development of gallium nitride (GaN) LEDs that emit high illumination of a blue/green light, the full-color LED display, white light LED, and LED's for traffic signals have all been introduced into the market. However, compared with traditional light sources, LED devices require more precise current and heat management. For example, the low thermal conductivity of sapphire usually creates high serial thermal resistance in an LED device.
Flip-chip LED devices are developed to improve the heat dissipation and current diffusion of conventional LED devices. For example, flip-chip LED devices may include a surface mount substrate, such as a silicon substrate, to improve thermal conductivity, especially in high power applications. In addition, layout of LED dies in flip-chip LED devices is usually designed to improve current diffusion and distribution. For example, the layout of the LED dies is designed such that patterned metal lines of p-electrodes and n-electrodes are utilized for conducting current. Furthermore, the p-electrodes and n-electrodes are usually disposed around the lateral surfaces of the LED dies, or both p-electrodes and n-electrodes are disposed in a common area.
FIG. 1 shows a layout of p-electrodes and n-electrodes in a conventional LED device 100, according to a conventional design. LED device 100 may include multiple p-electrodes and n-electrodes arranged in lines. For example, as shown in FIG. 1, LED device 100 may include two lines of n-electrodes at the left and right edges respectively, and three lines of p-electrodes in between. The p-electrodes and n-electrodes are arranged alternately such that each p-electrode (e.g., p-electrode 101) is on a central line between two adjacent n-electrodes, e.g., n-electrodes 107 and 108 and each n-electrode (e.g., n-electrode 108) is on a central line between two adjacent p-electrodes (e.g., p-electrodes 101 and 104). However, the distances between the closest p-electrodes and the n-electrodes are not constant. For example, the distance between p-electrode 101 and n-electrode 107 is different from the distance between p-electrode 101 and n-electrode 109 or 110. Further, the distance between n-electrode 108 and p-electrode 104 is different from the distance between n-electrode 108 and p-electrode 105 or 106.
Although LED device 100 as shown in FIG. 1 may effectively improve the current and heat management compared to conventional LED devices, it may nevertheless be sub-optimal. For example, due to the non-uniform distances, the paths of applied current distributed within LED device 100 have different lengths, and the intervals between the electron current paths are also different. Consequently, the internal serial resistances along the current paths are different. Therefore, different potential differences may be formed at different electrode pairs and non-uniform current distributions will occur in LED device 100. For example, the electrode pair 101-104 may have a different potential difference from the electrode pair 101-105. Such current diffusion difficulty and distribution non-uniformity may cause reduction in brightness and light emission efficiency of GaN blue or green LEDs.
Forming a transparent current diffusion layer on the top surface of a p-GaN layer may improve the current distribution in an LED device to some extent. With such a structure, the current may inject into the current diffusion layer after passing through the metal electrode. However, the current density in the area under the metal electrode remains higher than that under the current diffusion layer, and most current fluxes jam in the area under the metal electrode. Therefore, there is a need to further improve the contact resistances between the current diffusion layer and p-GaN layer.
The apparatus and method of the present disclosure are directed towards overcoming one or more of the constraints set forth above.